1. Field of the Invention
The present invention relates to a fixing device for fixing an image to a recording medium, an image forming apparatus including the fixing device, and a method of detecting an abnormality of the fixing device.
2. Description of the Related Art
With fixing devices used in image forming apparatuses developed in recent years, there has been a danger that a fire breaks out because of the temperature of a heating roller continuing to rise when the heating roller is entwined with recording sheet. This is because a power source for a heater heating the heating roller, which forms a fixing roller, fails to be turned OFF for the reason that the temperature detected by a temperature detecting member remains low in spite of the heating roller heated to a predetermined fixing temperature by the heater. In order to avoid such a danger, a method has been proposed which includes providing two temperature detecting members at a recording sheet passage point and at a point other than the recording sheet passage point, and detecting occurrence of entwining with recording sheet by utilizing the difference between the temperatures detected by the two temperature detecting members.
However, since such a fixing device requires two temperature detecting members, spaces for mounting the two temperature detecting members have to be secured and, in addition, wiring becomes complicated. Further, since the occurrence of entwining with recording sheet cannot be detected until the difference between the temperatures detected by the two temperature detecting members takes on a predetermined value or more, the fixing device involves a problem that it takes a relatively long time to detect the occurrence of entwining with recording sheet.
With respect to these problems, a fixing device capable of controlling the temperature of the heating roller by detecting the temperature with a single temperature detecting member while detecting the occurrence of entwining with recording sheet, is known (see Japanese Patent Laid-Open Publication No. HEI 07-287473 for example).
The fixing device described in Japanese Patent Laid-Open Publication No. HEI 07-287473, however, cannot detect abnormalities including a harness contact failure and a software control failure. These abnormalities make it impossible to control the temperature of the heating roller.
The present invention intends to provide a fixing device capable of detecting various troubles related to the heating roller temperature control thereby obviating an accident such as a fire.